A knowledge of the thermal properties of biomaterials has long been considered important to researchers and others interested in increasing man's understanding of the nature of materials and their thermal interactions, as well as to designers of equipment and systems in which the thermal characteristics of the materials used therein or operated thereon are of significance. For example, important information concerning biological materials, such as human and animal tissues, can be obtained from knowledge of the thermal properties thereof.
Thus, it is known that biomaterials are capable of heat transfers by virtue of a temperature gradient, such heat transfer capability being especially important in living biomaterials because the state of life thereof, for example, may depend on the maintenance of a specific temperature level. Heat transfer by conduction is usually most important in determining the heat transfer within the biological medium and such heat transfer is best characterized in the steady-state by the thermal conductivity, k, of the medium and in the non-steady state of its thermal diffusivity, .alpha.. Since there is no presently known method of determining k and .alpha. of a biomaterial from a knowledge of some other fundamental property or properties thereof, it is necessary to devise appropriate processes and apparatus to measure k and .alpha. in some appropriate manner. Accordingly, there has been an increasing utilization, particularly in medical research and clinical laboratories, of processes which require heat transfer through biological materials, such as in cryobiology (e.g., cryosurgery), in tissue and organ preservation, and in frostbite studies, for example. Other procedures which are heat transfer dependent and, thus, require a knowledge of thermal properties include clinical applications of ultrasonic wave energy, microwave energy and laser beam energy in both diagnostic and therapeutic operating modes.
Such processes require more extensive and more reliable information concerning the thermophysical properties of such materials and, in particular, information concerning the thermal conductivities and thermal diffusivities thereof which permit the determination of temperature distributions, heat transfer rates and, in turn, the flow rates of fluids through the biological medium. It is particularly important, for example, to monitor the flow rate of blood through tissue so that flow disturbances can be monitored and corrective action taken in cases where maldistribution of blood flow in a patient would have unfavorable and possibly fatal consequences.
Techniques which have been applied to the measurement of properties of biological materials have included both invasive and non-invasive techniques. A general summary of such techniques and the limitations thereof is presented in the text, Annual Review of Biophysics and Bioengineering, "Theory, Measurement and Application of Thermal Properties of Biomaterials, " H. Frederik Bowman et al., pp. 43-80, Vol. 4, 1975. While non-invasive techniques can provide information on thermal properties, they are necessarily limited to regions near the surface of the materials and, in order to obtain reliable information on thermal properties below the surface, invasive techniques are required. Such invasive techniques involve the implantation within the specimen material of heat sources (or sinks) which may also serve as temperature sensors. Probes which have been utilized for this purpose include the thermal comparator, the heated thermocouple and the heated thermistor. Up to now, however, no successful thermistor probe method and apparatus have become available which can provide realistic measurements of such thermophysical properties because of the limitations inherent in the thermal models which have heretofore been used in the analysis of the structure and operation of the probes and the media into which they are inserted.
For example, in the article "A Method for the Measurement of Thermal Properties of Biological Materials" by J.C. Chato, ASME Symposium on Thermal Problems in Biotechnology, LC No. 68-58741, 1968, Chato discusses the use of a thermistor probe, particularly in studying the thermal properties of biomaterials, and suggests that the probe technique has potential for measuring not only the thermal conductivity (k) thereof but also the thermal inertia (i.e., .sqroot.k.rho.c ) and the flow rate (.omega.) of blood as well. In utilizing the functional relationship between input power and probe temperature used in such technique, Chato assumed the thermistor bead to be a lumped thermal mass and using such assumption solved the heat conduction equation for the surrounding medium only. The Chato approach assumed a constant bead surface temperature at all times greater than zero and equal to a spatially uniform temperature rise in the bead.
While in principle the application of the solution of the heat conduction equation to the experimental data was expected by Chato to yield thermal conductivity and thermal inertia values, the assumption of a thermal model in which the thermistor bead was treated as a lumped thermal mass and the solving of the heat conduction equation solely for the surrounding medium was found by Bowman to be inadequate. Because of the inherent limitations in the thermal model, no meaningful measurements were able to be made of the desired thermal properties using the Chato techniques and, accordingly, the technique proved unsuccessful.